The present invention relates to a method for reconstruction of a plant for transmitting electric power, which comprises a direct voltage network for High Voltage Direct Current (HVDC) and two stations connected thereto with line commutated valves with thyristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks connected to the respective station thereof. The reconstruction aims at connecting further such stations to the direct voltage network while enabling feeding of electric power in an optional direction in each of the stations with the respective valve either in rectifying operation for feeding electric power from the alternating voltage network to the direct voltage network or with the valve in an inverter operation for feeding from the direct voltage network to the alternating voltage network. This invention also relates to a plant for using the present method.
The definitions above comprise not only the case, in which stations newly connected to the direct voltage network are connected to the direct voltage network already there, but these definitions are also intended to cover the case in which the direct voltage network is prolonged so as to enable connection of additional stations thereto, as would most often be the case. Also the case of a "tying together" of two direct voltage networks already existing for connection of more than two stations to the direct voltage network in such a reconstruction is intended to be covered by the definitions above.
There is sometimes interesting to reconstruct a so-called two-terminal-HVDC-line, i.e. a plant defined above with two stations for feeding electric power into the direct voltage network and draining electric power from the direct voltage network at the stations. It may then be desired to connect a third station to the direct voltage network, for example when a new such station is built on an island for taking care of the energy supply there, in which it is then desired to have a full freedom to feed power over the direct voltage network between the different stations in an arbitrary, desired direction. The thyristor and by that the valves of such two terminal plants of the type already known of this type can only conduct current in one direction, so that a changing of the direction of the transmission of power between the two stations then is made by changing the voltage polarity of the direct voltage network. However, it is not possible to add a conventional station to such a plant and obtain a full right of option with respect to the power feeding directions, since depending upon the arrangement of this valve, the current direction is then determined to always give a certain power feeding direction with respect to the power feeding direction of the other stations. Thus, is the valve thereof turned in the same direction as the valve of a certain of the stations, the power from the station added will always be fed in the same direction as in the first station. Accordingly, a much more extensive reconstruction of an existing plant with two stations is required, which will be very difficult to carry out and very costly, since a lot of existing equipment has to be changed, and new conduits and switch gears, low voltage as well as high voltage, are to be added for making possible to switch the valve and by that enable optional current direction. A considerable modification of existing control equipment in all the stations is then also required, which generates high costs. The valve halls already existing have also to be reconstructed as a consequence of the increased insulation distances required in and around the different valves, as well as the very expensive transformers are reconstructed or exchanged.
It is illustrated in FIG. 1 how such a reconstruction could take place in a plant already known, in which 1 designates a direct voltage network for High Voltage Direct Current, which here is unipolar, but it could just as well be bipolar with an earth return circuit, in which then in a known way two valves connected in antiparallel a station could be arranged and connected to a pole line each. It is shown here how three stations 2, 3, 4 are connected to the direct voltage network, in which these stations are summarised with the symbol for a valve 5, 6 and 7, respectively. Each such valve consists of line commutated thyristors, and an alternating voltage network 8 is connected to each station through a transformer 9, which for the sake of simplicity has only been shown for the station 2. By providing the stations 3 and 4 with switch gears with breakers 10-17 the power at a given voltage polarity of the direct voltage network 1 may be led to or away therefrom, depending upon the position of the breakers. The current of the direct voltage network 1 will always flow from the direct voltage network through the station 2, and when a positive voltage potential, which falls in the direction towards this station, is on the direct voltage network the valve 5 will function as inverter and transmit power from the direct voltage network to the alternating voltage network 8, while when the direct voltage network is on negative potential, which becomes more negative in the direction to the station 2, power could be fed in the opposite direction. By, for example, closing the breakers 10 and 13, but keeping the breakers 11 and 12 open, the station 3 will be in the same operation position as the station 2, in which power then will be fed to or from both thereof depending upon the polarity of the direct voltage network 1. Thus, the feeding direction will through one of the stations 3 and 4 be changed by reversing the polarities of the valve 6 and 7, respectively, so that in the case described above the breakers 11 and 12 are closed and the breakers 10 and 13 opened and, as the result, the current is led towards the direct voltage network. This solution to the problem to add further stations to a HVDC-line with two stations gets very costly as mentioned above due to a demand of supplemental control equipment and reconstruction of the valve halls as a consequence of changed insulation distances changed.